1. Field of the Invention
The present invention relates to an image forming apparatus including: a feed mechanism including a motor that is to be driven with supply of a power thereto, and a feeder that is to be moved by the motor for feeding a medium in a feed direction; a recording head operable to eject an ink toward the medium so as to form an image on the medium; a feed controller operable to control the motor so as to move the feeder to a target operating position that causes the medium to be positioned in a desired recording position; and an ink ejection controller operable to cause at least one ink ejection portion of the recording head to eject the ink therethrough toward the medium.
2. Discussion of Related Art
As a kind of image forming apparatus, there is known an inkjet printer of serial type including: (a) a feed mechanism including a motor that is to be driven with supply of a power thereto, and a feed roller that is to be rotated by the motor for feeding a medium in a feed direction; (b) a recording head operable to eject an ink toward the medium so as to form an image on the medium; (c) a carriage carrying the recording head; (d) a carriage driver operable to move the carriage in a main scanning direction perpendicular to the feed direction; (e) a feed controller operable, upon reception of a command requesting the medium to be fed to a desired recording position, to control the motor so as to move the feeder to a target operating position that causes the medium to be positioned in the desired recording position; and (f) an ink ejection controller operable, upon positioning of the medium in the desired recording position, to cause at least one ink ejection portion of the recording head to eject the ink therethrough toward the medium. The feed controller is operated to control the motor so as to intermittently move the feeder while the recording head forms the image on the medium, so as to position the medium in the desired recording position by each of successive feed motions of the feeder. The ink ejection controller is operated, based on data representative of the image to be formed, to cause the at least one ink ejection portion of the recording head to eject the ink therethrough while the carriage is being moved by the carriage driver in the main scanning direction after each of the successive feed motions of the medium.
In the above-described inkjet printer in which the recording medium has to be positioned in the desired recording position by each of the successive feed motions of the feed roller, if the recording medium is positioned by each feed motion in a position deviated from the desired recording position, white-colored or dark-colored extraneous lines are likely to appear on the formed image, resulting in poor quality of the image. For preventing such an undesirable appearance of the lines, in the inkjet printer, commonly, the feed roller is controlled by detecting or monitoring an operating angular position of the feed roller (i.e., position of the recording medium) through an operating angular position detector such as a rotary encoder in each of the successive feed motions of the feed roller.
Conventionally, when the recording medium is to be moved to a certain desired position, the motor is once accelerated and then gradually decelerated such that a rotational velocity of the feed roller is reduced to a sufficiently low value in proximity of the desired position. Then, supply of electric power to the motor is stopped at a point of time at which an actual position of the moved recording medium reaches a motor OFF position that is located before the desired position by a certain amount, so that the feed roller is rotated by inertia for a while and then eventually stopped, as shown in FIG. 12A.
In this arrangement for positioning the recording medium in the desired position, as long as the rotational velocity of the feed roller at the motor OFF position is constantly controlled to be a predetermined value, an amount a of the inertial rotation of the feed roller can be held constant, whereby the feed roller can be stopped in a target operating position that causes the recording medium to be positioned in the desired position. However, due to a torque fluctuation of the feed mechanism (including the motor, feed roller, and power transmission member connecting the motor to the feed roller), if the rotational velocity of the feed roller at the motor OFF position is made lower than the predetermined value, as indicated by broken line in FIG. 12A, the amount a of the inertial rotation of the feed roller after the stop of the power supply to the motor is reduced, whereby the feed roller is likely to be stopped before the target operating position.
Commonly, the above-described motor driving the feed roller is provided by a DC motor. Due to its constructional character, a torque of the DC motor is fluctuated rather than being constant during each one rotation of a drive shaft thereof. That is, the DC motor has a so-called “cogging” by which the torque is cyclically fluctuated. Consequently, the rotation of the feed roller is affected by the cyclic fluctuation of the torque of the DC motor (see FIG. 12B).
Where a reduction in the torque of the DC motor due to the cyclic fluctuation of the torque is caused in vicinity of the motor OFF position, the amount α of rotation of the feed roller after the stop of the power supply to the motor until the stop of rotation of the feed roller is reduced, thereby making it impossible to stop the feed roller in the target operating position, failing to position the recording medium in the desired position.
For preventing such a problem, there is an arrangement, as disclosed in U.S. Pat. No. 6,702,492 (corresponding to JP-2002-128313A), in which a minimum controllable operating amount of the feed roller is adapted to be integer number of times as large as a cycle length of the cogging of the motor, such that the amount α of the inertial rotation of the feed roller after the stop of the power supply to the motor until the stop of rotation of the feed roller is constantly held in a predetermined amount. In the disclosed arrangement, a gear ratio between the motor and the feed roller is set to be an amount that causes the minimum controllable operating amount of the feed roller to be integer number of times as large as the cycle length of the cogging.
However, in the above-described arrangement, it is not possible to finely adjust control parameters representative of a relationship between the rotation amount of the motor and the feed amount of the recording medium, for accurately control the feed amount of the recording medium in presence of some erroneous variations in dimensions of the feed mechanism such as a diameter of the roller feeder and dimensions of gears, belt or other components constituting the feed mechanism. Thus, the arrangement has a problem that the components of the feed mechanism are required to have extremely high dimensional accuracy, increasing a cost required therefor.